Electron sources are used for industrial and scientific purposes in a wide range of applications such as electron beam welding, medical device sterilization, x-ray imaging, electron microscopy, electron beam lithography, polymer cross-linking, cargo scanning and sterilization. The lack of high-power, robust electron sources that can operate in harsh environments has limited the adoption of electron accelerators for energy and environmental processes such as sterilization of water, wastewater and sludge, decontamination of gas streams, food decontamination, and the polymerization of asphalt roadways. Additionally, precisely focused electron beams are required for state-of-the-art ultra-fast transmission electron microscopy (UTEM) which promises to be one of the most powerful tools for dynamic investigation on the nano-scale. Other RF devices such as gridless Inductive Output Tubes (IOTs) or klystrode type devices benefit from an advanced electron source as well. The ability to emit continuous or finely controlled low emittance electron pulses without a high-power modulator or grid enable greater simplification of electron injectors for accelerator systems.